Microporous membranes, i.e., thin sheets of material having pores from a few micrometers in diameter down to about 0.05 .mu.m in diameter, have long been known. Such membranes may be made out of many different materials such as naturally occurring polymers, synthetic polymers, and ceramics. Depending upon the material from which the membrane is made, its wetting characteristics may differ greatly.
Liquid-repelling membranes often find use in filtration of gases, venting filters, and gas vents. Such membranes are herein referred to as "hydrophobic" even though, as will be clear from the context, liquids other than water (surface tension about 72.4 dynes/cm) are repelled by such membranes. Hydrophobic membranes are effective in these applications because they will allow gases and vapors, which have low surface tensions, to pass through the membrane while excluding materials with high surface tensions, for example, many liquids, from the membrane. For example, a gas filter will be effective if it allows only gas to pass but will not allow drops of liquid such as steam condensate, pump oil droplets, or other mists to penetrate and fill (and thereby block) the pores of the filter.
Frequently, these situations are encountered by filters used to sterilize the air feed to a biological fermentor. These filters are often sterilized after installation by exposure to steam. Should steam condensate penetrate and remain in the filter membrane the membrane would become blocked to further steam and subsequent air flow during use. Similarly, if water or oil droplets from air compressors or other sources should penetrate the filter membrane during use, the membrane would become blocked and reduce air flow during further use.
Hydrophobic membranes are also used in vent filters. In this application they protect the cleanliness of a liquid inside a vessel while permitting the vapor in the head space of that vessel to flow freely, both into and out of the vessel, as that vessel is filled and/or depleted of its contents. It frequently occurs that the liquid in such a vented vessel contacts the filter membrane in vent filters due to splashing or overfilling the vessel. If the membrane is wetted upon contact with the liquid, the liquid will penetrate the membrane and fill its pores, eliminating free flow of gas through the filter. Restrictions in flow through the vent will cause reduced drainage of liquid from the vessel and in some instances, collapse of the vessel itself. To perform effectively in such applications, the membrane must not be wetted by the liquid upon contact with it.
Hydrophobic membranes are also used in gas venting applications where the membrane is in constant contact with a liquid containing bubbles of gas. In such applications the membrane must serve as a barrier to the liquid and contain it while permitting the gas in the liquid to escape through the membrane. The membrane also serves as a filter to protect the contained liquid from contamination from the environment to which the gas escapes. In such applications, too, the membrane must not be wetted by the liquid upon contact with it. If the liquid were to wet the membrane, the liquid would penetrate the membrane, flow through it, and be lost from its contained system. Furthermore, the membrane would then be blocked by the liquid and no longer will be permeable to gas. It would then be unable to function as a vent.
In many of the above applications the membrane must behave as a sterilizing barrier; that is, it must be completely bacterially retentive. Not only must the membrane itself have such a small pore size that it can perform such a function, but also the device must be completely sealed so that it will not leak or bypass. To qualify for use in such critical applications it must be possible to test the device in order to determine that there are no faults and to ensure its ability to function.
Most frequently this is done by means of tests such as "bubble point" or "pressure hold" tests. These tests are referred to as integrity tests and are well known to those skilled in the art. They make use of the capillary properties of microporous membranes when fully wetted with a suitable test liquid. To be effective in such gas and vent filter applications, the filter membrane of choice must completely reject the liquids with which it may come in contact during use. However, it must also be able to be fully wetted by a suitable liquid used for testing the integrity of the filter or device. The wetting characteristics of the membrane must therefore be controlled carefully so that the membrane will not be wetted by most liquids encountered during fluid handling operations yet will be easily and completely wetted by special fluids used for carrying out integrity tests.
The ability of a solid surface to be wetted upon contact with a liquid depends upon the surface tension of the liquid and the surface free energy of the solid surface. In general, if the surface tension of the liquid is less than the surface free energy of the solid surface, the surface will be spontaneously wetted by that liquid. An empirical wetting property of a porous matrix, its critical wetting surface tension (CWST), can easily be determined. The CWST of a porous matrix such as a microporous membrane may be determined by finding the liquid having the highest surface tension within a homologous series of inert liquids which will spontaneously wet the porous matrix. For the purposes of this disclosure a porous membrane being "spontaneously wetted" means that when such a membrane is placed in contact with a liquid that liquid is drawn into the porous structure of the membrane within a few seconds without the application of external pressure. Liquids having surface tensions below the CWST of the porous matrix will wet it; liquids having surface tensions above the CWST of the porous matrix will not wet it and will be excluded.
Membranes made of materials which contain only non-polar groups and which have low critical surface tensions are not spontaneously wetted by liquids having high surface tensions, for example, water and most aqueous solutions. Microporous membranes made of non-polar materials such as polypropylene, poly(vinylidene fluoride), and polytetrafluoroethylene are available from Celanese, Millipore, and Gore Co., respectively. These membranes are naturally hydrophobic and are not spontaneously wetted by water. Such membranes have CWSTs ranging from about 28 to about 35 dynes/cm, depending on the material from which the membrane is made.
The microporous membranes which will be most useful as air filters, vent filters, and air vents will be those membranes which have as low a CWST as can be obtained in order to avoid penetration of the pores of the membrane by liquids with which they come in contact when in use. Currently the microporous membranes commercially available which have the lowest CWST are microporous membranes made of polytetrafluoroethylene, or PTFE. Such membranes are sold by the Gore Company and by Sumitomo Electric, Incorporated, among others and are available having a limited number of pore sizes ranging from 0.05 .mu.m to 1 .mu.m.
The CWST of these PTFE membranes is about 28 dynes/cm, which means that liquids having surface tensions equal to or lower than this value will spontaneously wet these membranes. Liquids having surface tensions higher than 28 dynes/cm will not spontaneously wet the membranes. Therefore, these PTFE membranes will function effectively in vents, vent filters, and gas filters as long as the membrane is not contacted with liquids having surface tensions of 28 dynes/cm or less. However, many aqueous solutions, chemicals, and many solvents and oils have low surface tensions and will wet PTFE membranes, either spontaneously or if modest pressure is accidentally applied. If the surface tension of the liquid is above the CWST of the membrane, the liquid can be forced to wet the membrane under pressure. The amount of pressure required is small if the difference between the surface tension of the liquid and the CWST of the membrane is small.
A microporous material which has a CWST much less than that of PTFE membranes would make accessible membranes that could be used in applications involving a greater variety of chemicals and fluids. In addition, while PTFE membranes are commercially available, they are very expensive and are difficult to use in an economical manner. PTFE membranes are not available having all desired pore sizes. Furthermore, PTFE is degraded severely by radiation, making it an undesirable material for use in vents and filters for sterile medical applications, where sterilization by means of radiation is the most economical and safe method of sterilizing these products after manufacture.
It is an object, therefore, of this invention to provide a microporous membrane which has a CWST controlled to a value significantly less than that of a membrane made of PTFE and yet which is above that of certain liquids useful as integrity test fluids, said membrane being economical to produce and capable of being made having a wide range of pore sizes in a controlled manner from materials resistant to damage by high doses of radiation, particularly doses associated with sterilization.
It is also an object of this invention to provide devices for processing fluids which use such a hydrophobic membrane to separate a gas but retain a liquid.
It is a further object to provide methods of using the hydrophobic membranes of the invention, for example, in gas filtration/drying, as a venting filter, or as a gas/liquid separator.